A few years ago it was more frequent but when PWT owned the route it was faster and more frequent but more expensive. Once-an-hour service shows that GO has no interest in providing Toronto residents with the airport service it acquired.
What are you talking about? PWT?
GO bus service in and out of YYZ (York Mills - Brampton) has been hourly seven days a week, 365 days a year, since... I don't know.
Now, you might find an extra bus or two at peak periods, and you might not find a bus early sunday mornning, but the basic schedule hasn't changed in at least 25 years.